Various studies have found that the noise signature produced by traffic on any moderate to high speed roadway is composed of the following elements: 1) Tire to pavement noise due to contact between the rubber surface of the tread on a tire and the surface of the road itself; 2) Aerodynamic noises; 3) Engine/exhaust noise due to the combustion process; and 4) Transmission and other rotating components within the driveline. Typically for an automobile that is in good operating condition with a properly functioning exhaust system, the overwhelming majority of noise is produced by the tire to pavement contact. The problem is further aggravated by the fact that for many high speed highways and interstates, codes require the use of transverse grooves to aid in shedding water from the surface to minimize hydroplaning. Because of the typical highway speeds (55 to 70 MPH), and the regular spacing of the grooves, the action of the tire tread is to have a portion of the contact patch actually alternately contact and not-contact the road surface. This action causes a dominant noise frequency component that is proportional to the speed of the tire and the regular spacing of the rain channel grooves. This tone most usually manifests itself as a whistle or whining noise, which is actually comprised of a relatively narrow spectrum of signals centered around a single dominant component.
The dominant tone can be defined as the fundamental frequency of oscillation of the tire to road interface. This dominant tone frequency can be calculated from the following relationship: Tone (Hz)=(MPH*17.6)/Groove Spacing (in inches).
For example, for a vehicle traveling at 60 MPH and a groove spacing of 1 inch, the dominant frequency produced is 1056 Hz. FIG. 1A illustrates in a cross sectional view such groove spacing.
The spectral energy density profile for a single tone would be represented by the diagram FIG. 2A.
It is apparent that most of the acoustic noise energy is concentrated around the dominant tone frequency which is a function of the line/groove spacing and the vehicle speed. Previous efforts such as in U.S. Pat. Nos. 4,105,458 and 4,396,312 have been directed at the road surfacing materials used. Any noise reduction was more or less a side effect. However, the present invention is directed specifically to noise reduction irrespective of the choice of materials used for road surfacing. The application of random transverse grooves has been addressed by the North Dakota Department of Transportation (NDDOT), Materials and Research Division, “Evaluation of Tining Widths to Reduce Noise of Concrete Roadways Final Report”, and LEE etal. (Korean Patent 2004005583). In both these applications acknowledgement has been made as to the effectiveness of random patterns but the apparent benefits are less than optimal due to the insufficient pattern repetition lengths and due to the fact that those patterns used were not generated by a random mathematical process. The narrow spectrum gains of short or repetitive patterns are of limited benefit.